Battery

ABSTRACT

A battery comprising a positive electrode ( 10 ), a negative electrode ( 20 ) and a separator ( 31 ) provided interposed between the positive electrode ( 10 ) and the negative electrode ( 20 ), characterized in that at least one surface of said separator ( 31 ) is bonded to said positive electrode ( 10 ) or negative electrode ( 20 ) via a porous resin layer ( 41, 33 ) containing a solid filler. This battery can provide a battery which exhibits a high energy density and an excellent cycle life performance even if the electricity-storing element is received in a flexible material case.

TECHNICAL FIELD

[0001] The present invention relates to a battery and particularly to abattery to be incorporated in small-sized electronic equipments.

BACKGROUND ART

[0002] In a battery having a metallic can as a container, it has beenheretofore practiced to press the electrodes under a predeterminedpressure and make the distance between the electrodes even. This isbecause that, when uniform distance between the electrodes along thesurface of the electrodes is usual in that the electrode reactionproceeds uniformly all over the electrodes, expecting a prolonged life.

[0003] In recent years, a thin battery using a container of, e.g., ametal-resin laminate film rather than metallic can has appeared. Thisbattery comprises an electricity-storing element made of a positiveelectrode, a separator and a negative electrode received in an airtightbag obtained by adhering a laminate film at the edges thereof.

[0004] However, since this type of a battery comprises, as a batterycontainer, a container made of a flexible laminate film rather than ametallic can, the electrodes cannot be pressed by the pressure of thebattery container. Thus, the distance between the electrodes isununiform, causing a remarkable drop in the capacity during charge anddischarge cycles.

[0005] Thus, Japanese Patent Application Laid-Open No. 1998-302843proposes that the separator and the electrodes be bonded to each otherwith an adhesive. In accordance with this proposal, the distance betweenthe electrodes can be kept constant even without any pressure of thebattery container, causing the electrode reaction to proceed uniformlyall over the electrodes and hence giving a prolonged life. As theadhesive used, ethylene glycol dimethacrylate, methyl methacrylate orthe like is dissolved in N-methylpyrrolidone or the like.

[0006] However, when such an adhesive is used, a dense adhesive layer isformed on the surface of the electrodes. Accordingly, this layer ofadhesive was disadvantageous in that it prevents the movement of theelectrolyte across the space between the electrodes, causing an energydensity drop.

[0007] It is therefore an object of the present invention to provide abattery which exhibits a high energy density and an excellent cycle lifeperformance even if the electricity-storing element is received in aflexible material case.

DISCLOSURE OF THE INVENTION

[0008] The battery of the present invention is a battery comprising apositive electrode, a negative electrode and a separator providedinterposed therebetween, wherein at least one surface of said separatoris bonded to said positive electrode or negative electrode via a porousresin layer comprising a solid filler.

[0009] In the battery of the invention, since the electrode and theseparator are bonded to each other with a porous resin layer asmentioned above, the distance between the electrodes can be keptconstant even if the battery container is so flexible that the pressureof the battery container is not sufficient. Accordingly, even whensubjected to repeated charges and discharges, the battery of theinvention shows no capacity drop and thus exhibits a prolonged life.

[0010] Further, since the resin layer is made porous by the addition ofa solid filler, the electrolyte can move across the space between theelectrodes through the pores formed in the resin layer, enhancing theenergy density.

[0011] The thickness of the resin layer is preferably from 1 μm to 10μm. This is because the energy density of the battery can be enhancedwhen the thickness of the resin layer falls within this range. In otherwords, when the thickness of the resin layer falls below 1 μm, theadhesion between the electrode and the separator becomes insufficient.Therefore, it is likely that when the electricity-storing element isreceived in the battery container or the battery is in use, the distancebetween the electrodes can become ununiform, causing a capacity dropwith the repetition of charge and discharge. On the contrary, when thethickness of the resin layer exceeds 10 μm, the distance between theelectrodes becomes too long, causing an energy density drop.

[0012] The thickness of the separator is preferably not greater than 25μm. This is because the energy density of the battery can be enhancedwhen the thickness of the separator falls within this range. In otherwords, when the thickness of the separator exceeds 25 μm, the distancebetween the electrodes becomes too large, causing an energy densitydrop.

[0013] The resin to be used in the resin layer is not specificallylimited but preferably comprises at least one member selected from thegroup consisting of polyethylene, polypropylene, poly(vinylidenechloride), poly(vinylidene fluoride), poly(ethylene oxide) andpolyacrylonitrile.

[0014] Alternatively, the resin to be used in the resin layer preferablycomprises at least one member selected from the group consisting ofcopolymer of vinylidene fluoride and hexafluoropropylene, copolymer ofvinylidene fluoride and chlorotrifluoroethylene, copolymer of vinylidenefluoride, hexafluoropropylene and tetrafluoroethylene, copolymer ofvinylidene fluoride and tetrafluoroethylene and copolymer ofhexafluoropropylene and tetrafluoroethylene.

[0015] The solid filler to be used in the resin layer preferablycomprises a ceramic powder made of primary particles having an averagediameter of from 5 to 100 nm.

[0016] This is because, when a resin solution containing a ceramicpowder falling within this range is dried, the resin solution isadsorbed to the ceramic powder during drying. A lower amount of theresin solution exists less in the portion other than the portion havingthe ceramic powder present therein and thus forms pores when dried,rendering the resin layer porous. When the particle diameter of theceramic powder exceeds 100 μm. the resin is adsorbed less to the ceramicpowder, making it impossible to make the resin layer uniformly porousand hence causing a capacity drop and a resistance rise.

[0017] The ceramic powder is not specifically limited but preferablycomprises at least one member selected from the group consisting ofalumina, silica, titania and zirconia. This is because these materialsare all excellent in resistance to organic electrolyte.

[0018] The specific surface area of the solid filler is preferably fromnot smaller than 50 m²/g to not greater than 500 m²/g. This is becausewhen the specific surface area of the solid filler falls below 50 m²/g,the resin is adsorbed less to the ceramic powder, making it impossibleto make the resin layer uniformly porous and hence causing a capacitydrop and increase of the resistance. This is also because when thespecific surface area of the solid filler exceeds 500 m²/g, the amountof a solvent to be adsorbed to the ceramic powder increases during thepreparation of a paste of the resin, the ceramic powder and the solvent,making it difficult to form a uniform resin layer and hence lowering theadhesion strength, which deteriorates the cycle life performance.

[0019] A part of the resin layer preferably penetrates into the surfacelayer of the positive electrode and negative electrode. This is because,when a part of the resin layer penetrates into the surface layer of thepositive electrode and negative electrode, the separator can be firmlybonded to the positive electrode and negative electrode to keep thedistance between the electrodes constant. Thus, when subjected torepeated charges and discharges, the battery undergoes no capacity dropand hence exhibits a prolonged life.

[0020] The battery of the invention may be applied to any type such ascylindrical battery, prismatic battery, sheet-shaped battery, laminatedbattery, coin-shaped battery and pin-shaped battery. The shape of thebattery of the invention is not specifically limited, but the positiveelectrode, the negative electrode and the separator are preferablyreceived in a flexible material case. The distance between theelectrodes can be difficultly kept constant particularly when thebattery container is flexible. Even in such a case, the presentinvention makes it possible to keep the distance between the electrodesconstant, causing no capacity drop and hence giving a prolonged life.

[0021] The battery of the present invention can be widely usedregardless of which it is of primary type or secondary type.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is an enlarged sectional view of one embodiment of thepositive electrode of the present invention;

[0023]FIG. 2 is an enlarged sectional view of one embodiment of thenegative electrode of the present invention;

[0024]FIG. 3 is an enlarged sectional view illustrating one embodimentof the present invention wherein a separator is bonded to the negativeelectrode of the present invention;

[0025]FIG. 4 is an enlarged sectional view illustrating one embodimentof the present invention wherein a resin layer is formed on the positiveelectrode;

[0026]FIG. 5 is an enlarged view illustrating one embodiment of thepresent invention wherein the negative electrode and the positiveelectrode are bonded to each other;

[0027]FIG. 6 is a perspective view of one embodiment of theelectricity-storing element and the battery container of the presentinvention;

[0028]FIG. 7 is a perspective view of one embodiment of the battery ofthe present invention;

[0029]FIG. 8 is an electron microphotograph of the resin layer ofExample 1;

[0030]FIG. 9 is an electron microphotograph of the resin layer ofComparative Example 2;

[0031]FIG. 10 is a graph illustrating the relationship between theparticle diameter and the initial capacity and battery resistance; and

[0032]FIG. 11 is a graph illustrating the relationship between thespecific surface area and the capacity.

BEST MODE FOR CARRYING OUT THE INVENTION

[0033] Preferred embodiment of the present invention will be describedin further detail.

[0034] In order to confirm the effect of the present invention, alithium ion battery having the following specification was prepared. Theelectricity-storing element of this battery comprises a positiveelectrode, a separator, a negative electrode and a separator.

[0035] The positive electrode 10 comprised a positive composite 12retained on both sides of a current collector 11 made of an aluminumfoil having a thickness of 20 μm as shown in FIG. 1. The positivecomposite 12 was prepared by mixing 91 parts of a lithium cobaltcomposite oxide LiCoO₂ as a positive active material, 6 parts of apoly(vinylidene fluoride) as a binder and 3 parts of acetylene black asan electrically conducting material to make a paste. The composite 12was spread to both sides of the current collector 11, dried, and thenrolled to prepare the positive electrode 10. The positive electrode 10was then cut to a predetermined width. Thus, the positive electrode 10was then used in the form of belt.

[0036] On the other hand, as shown in FIG. 2, the negative electrode 20comprised a negative composite 22 retained on both sides of a currentcollector 21 made of a copper foil having a thickness of 10 μm. Thenegative composite 22 was prepared by mixing 92 parts of a graphitepowder having a specific surface area of 1 m²/g as a negative activematerial and 8 parts of a poly(vinylidene fluoride) as a binder, andthen properly adding N-methyl-2-pyrrolidone to the mixture to make apaste. The composite 22 was applied to both sides of the currentcollector 21, dried, and then rolled to prepare the negative electrode20. The negative electrode 20 was then cut to a predetermined width, andused in the form of belt.

[0037] As the separator, a porous polyethylene sheet having a porosityof 45% was provided interposed between the electrodes. The thickness ofthe separator was varied from 15 to 25 μm as set forth in Table 1 shownlater.

[0038] As the resin layer, a copolymer of vinylidene fluoride andhexafluoropropylene was previously emulsion-polymerized. The emulsion ofcopolymer was dehydrated to a powder, which was then dissolved inN-methyl-2-pyrrolidone. To the solution was then added to an aluminapowder to prepare a sticky paste. An alumina powder having a primaryparticle diameter of from 10 to 20 nm and a specific surface area of100±15 m²/g (the BET method) was used. The sticky paste used comprisedthe copolymer and the alumina powder at a mixing ratio of 1:1 by dryweight.

[0039] Subsequently, as shown in FIG. 3, the mixture was applied to oneside of a separator 31 to form a resin layer 33. Two sheets of theseparator 31 having the resin layer 33 formed thereon were prepared andbonded to the respective side of the negative electrode 20 before theresin layer 33 was dried. The bonding was carried out such that theresin layer 33 of the separator 31 was opposed to the negative electrode20.

[0040] Subsequently, as shown in FIG. 4, the same mixture as mentionedabove was applied to both sides of the positive electrode 10. Duringthis procedure, the spread amount of the mixture was adjusted such thatthe resin layer 41 formed on the positive electrode 10 had the samethickness as that of the resin layer 33 formed on the separator 31.Subsequently, as shown in FIG. 5, the negative electrode 20 having theseparator 31 bonded to both sides thereof and the positive electrode 10having the resin layer 41 formed on both sides thereof were laminatedand bonded to each other. The end of the current collector 11 of thepositive electrode 10 is not coated with the positive composite 12, anda leaf of aluminum lead 64 is ultrasonically welded thereto (see FIG.6). The end of the current collector 21 of the negative electrode 20 hasa region which is not coated with the negative composite 22, and a leafof lead 65 is ultrasonically welded thereto (see FIG. 6).

[0041] The electrodes were wound on a metallic core, and the core wasthen pulled out of the winding to produce an electricity-storing element61. Subsequently, the electricity-storing element 61 was received in abag-shaped container 63 made of a metal-resin laminate film as shown inFIG. 6. The container 63 is made of a metal-resin laminate film having athree-layer structure comprising a surface protective layer made of PET(poly(ethylene terephthalate)), a barrier layer made of aluminum and aweld layer made of PE (polyethylene). The metal-resin laminate film isfolded with the weld layer inside, and then welded at the bottom andsides thereof to form a bag. Finally, the bag is welded at the upperopening thereof.

[0042] Subsequently, an electrolyte was injected into the container 63which had the electricity-storing element 61 received therein. Theelectrolyte is a 1:1 (by volume) mixture of ethylene carbonate anddiethyl carbonate containing 1 mol/l of LiPF₆. Thus, a 650 mAh lithiumion battery 70 having an average discharge voltage of 3.7 V and a sizeof 3.8 mm (thickness)×35 mm (width)×62 mm (length) was obtained.

[0043] Batteries of Examples 1 to 6 were prepared from differentcombinations of the thickness D of separator and the thickness J ofresin layer as set forth in Table 1. As Comparative Example 1, a batteryfree of resin layer was prepared. As Comparative Example 2, a batterycomprising a resin layer but free of alumina powder therein wasprepared. The batteries of Examples 1 to 6 and Comparative Examples 1and 2 were each subjected to charge and discharge cycles, and thenmeasured for the capacity change. The results are set forth in Table 1.Referring to the charge and discharge cycle conditions, one cycleconsists of 3 hours of charging at a constant voltage of 4.2 V anddischarging to 2.75 V with a constant current of 1 CA (650 mA) . Thecapacity in Table 1 is represented relative to the discharge capacity of650 mA of the battery of Example 3 which has been charged for the firsttime as 100. TABLE 1 Compar- Compar- Example Example Example ExampleExample Example ative ative 1 2 3 4 5 6 Example 1 Example 2 Thickness Dof 20 20 20 15 20 20 25 20 separator (μm) Thickness J of 1.5 1.0 3.0 5.00.5 7.0 0.0 1.5 resin layer (μm) Kind of solid Alumina Alumina AluminaAlumina Alumina Alumina — — filler Capacity at 101.8 102.7 100.0 100.0103.9 85.5 100.0 42.5 1st cycle (%) Capacity at 96.9 97.1 94.3 94.5 82.777.0 77.0 — 100th cycle (%) Capacity at 93.2 92.5 90.0 91.8 69.0 72.532.2 — 200th cycle (%) Capacity at 91.1 90.4 84.7 88.8 — 69.0 — — 300thcycle (%)

[0044] As can be seen in Table 1, all the batteries of Examples 1 to 6exhibited a high initial capacity and maintained a high capacity evenafter 200 cycles. The batteries of Examples 1 to 4 and Example 6 stillmaintained a high capacity after 300 cycles.

[0045] On the contrary, the battery of Comparative Example 1, which isfree of resin layer, showed an initial capacity but showed a capacitydrop every repetition of charge and discharge cycle and then showed anextremely low capacity after 200 cycles. Thus, the batteries of Examples1 to 6 have electrodes and a separator bonded to each other with aporous resin layer and can keep the distance between the electrodesconstant even if the battery container is so flexible that the pressurefrom the battery container is not sufficient. These batteries wereconfirmed to undergo no capacity drop even after repeated charge anddischarge and exhibit a prolonged life.

[0046] The battery of Comparative Example 2, which comprises no aluminapowder incorporated in the resin layer, exhibited an extremely lowinitial capacity.

[0047] Subsequently, the resin layer 41 formed between the positiveelectrode 10 and the separator 31 and the resin layer 33 formed betweenthe negative electrode 20 and the separator 31 in the battery of Example1 were observed under electron microscope. FIG. 8 illustrates anelectron microphotograph of the resin layer 41 formed between thepositive electrode 10 and the separator 31. The resin layer wasconfirmed porous as shown in FIG. 8. Though not shown, the resin layer33 formed between the negative electrode 20 and the separator 31 wassimilarly confirmed porous.

[0048]FIG. 9 illustrates an electron microphotograph of the resin layerfree of alumina powder formed in the battery of Comparative Example 2.The resin layer in the battery of Comparative Example 2 was confirmednon-porous. It was thus confirmed that a resin layer filled with analumina powder as a solid filler is rendered porous. Accordingly, it wasconfirmed that the batteries of Examples 1 to 6 exhibit a higher energydensity than the battery of Comparative Example 2 and a high initialcapacity.

[0049] The effect of the average particle diameter of solid filler onthe initial capacity was then studied. In this test, batteries havingthe same structure as that of Example 1 except that the average particlediameter of the alumina powder were varied were prepared (see Table 2).In other words, the thickness of the separator was 20 μm and thethickness of the resin layer was 1.5 μm.

[0050] These batteries were each subjected to charge and dischargecycle, and then measured for initial capacity. The results are set forthin Table 2. Referring to the charge and discharging cycle conditions,one cycle consists of 3 hours of charging at a constant voltage of 4.2 Vand discharging to 2.75 V with a constant current of 1 CA (650 mA). InTable 2, the initial capacity is represented relative to that of thebattery of Comparative Example 1 as 100, and the electrical resistanceis represented relative to that of the battery of Comparative Example 1as 100.

[0051] As shown in Table 2 and FIG. 10, the batteries comprising analumina powder having an average particle diameter of from 5 to 100 nmexhibit a higher initial capacity and a lower resistance than that ofComparative Example 1 which is free of resin layer. TABLE 2 Averageparticle Initial capacity Battery diameter (nm) (%) resistance (%) 042.5 145 3 77.3 127 5 100.3 103 10 101.8 100 50 102.0 100 100 101.3 105150 97.0 118

[0052] The effects of the specific surface area of the solid filler onthe initial capacity and cycle life performance were then studied. Inthis test, batteries having the same structure as that of Example 1except that the specific surface area of alumina powder was varied wereprepared (see Table 3). In other words, the thickness of the separatorwas 20 μm and the thickness of the resin layer was 1.5 μm.

[0053] These batteries were each subjected to charge and dischargecycles, and then measured for initial capacity and capacity after 100cycles. The results are set forth in Table 3. Referring to the chargeand discharge cycle conditions, one cycle consists of 3 hours ofcharging at a constant voltage of 4.2 V and discharging to 2.75 V with aconstant current of 1 CA (650 mA). In Table 3, the initial capacity andcapacity after 100 cycles are represented relative to that of thebattery of Comparative Example 1 as 100, and the electrical resistanceis represented relative to that of the battery of Comparative Example 1as 100.

[0054] As shown in Table 3 and FIG. 11, the batteries comprising analumina powder having a specific surface area of from not smaller than50 m²/g to not greater than 500 m²/g exhibit a higher initial capacityand a longer life than that of Comparative Example 1 which is free ofresin layer. TABLE 3 Specific surface area Initial Capacity at (m²/g)capacity (%) 100th cycle (%) 28 72.9 66.2 55 100.8 95.8 108 101.8 96.9480 100.9 94.8 770 100.2 81.2

[0055] Industrial Applicability

[0056] As mentioned above, the present invention can provide a batteryhaving a high energy density and an excellent cycle life performance,even when the electricity-storing element is received in a case made ofa flexible material.

1. (Amended) A battery comprising a positive electrode, a negative electrode and a separator provided interposed therebetween, wherein at least one surface of said separator is bonded to said positive electrode or negative electrode via a porous resin layer comprising a solid filler, wherein said solid filler comprises a ceramic powder comprising primary particles having an average diameter of from 5 nm to 100 nm and the specific surface area of said solid filler is from not smaller than 50 m²/g to not greater than 500 m²/g.
 2. The battery as defined in claim 1, wherein the thickness of said resin layer is from 1 μm to 10 μm.
 3. The battery as defined in claim 1 or 2, wherein the thickness of said separator is not greater than 25 μm.
 4. The battery as defined in claims 1, 2 and 3, wherein said resin layer comprises at least one member selected from the group consisting of polyethylene, polypropylene, poly(vinylidene chloride), poly(vinylidene fluoride), polyethylene oxide and polyacrylonitrile.
 5. The battery as defined in claims 1, 2 and 3, wherein said resin layer comprises at least one member selected from the group consisting of copolymer of vinylidene fluoride and hexafluoropropylene, copolymer of vinylidene fluoride and chlorotrifluoroethylene, copolymer of vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene, copolymer of vinylidene fluoride and tetrafluoroethylene and copolymer of hexafluoropropylene and tetrafluoroethylene.
 6. (Deleted)
 7. (Amended) The battery as defined in claim 1, wherein said ceramic powder comprises at least one member selected from the group consisting of alumina, silica, titania and zirconia.
 8. (Deleted)
 9. The battery as defined in any one of claims 1 to 8, wherein said positive electrode, said negative electrode, said separator and said resin layer are at least partly impregnated with a non-aqueous electrolyte.
 10. The battery as defined in any one of claims 1 to 9, wherein a part of said resin layer penetrates into the surface layer of said positive electrode and said negative electrode.
 11. The battery as defined in any one of claims 1 to 10, wherein said positive electrode, said negative electrode and said separator are received in a flexible material case. 